The present invention is related to glasses for infrared applications, especially infrared athermal fluoride glasses whose optical path length change as a function of temperature change will become practically zero especially in the infrared region of 1 to 5.5 .mu.m, and optical parts as well as optical equipment utilizing these glasses.
In performing precise optical measurements, generally, the problem is that there is an optical system variation due to temperature change. As a typical example, the optical system may comprise a camera, especially camera lenses, for example, on board a space satellite, where a large temperature difference will occur between the portion irradiated by the sun light and the shaded portion. When a temperature difference occurs in a lens, it will appear as heterogeneity in refractive index and cause disorder in the image, and it MAY detrimentally affect the resolution capability of the camera--this has been observed in the past. Generally, the refractive indices of oxide system glasses rise with the rise in temperature. The temperature coefficient of refractive index, dn/dT is of the order of +10 to +2.times.10.sup.-6 /.degree. C. (Tryggve Baak, J. Opt. Soc. Am., 59 (1969) 851).
In conjunction with the refractive index change, there is a linear expansion coefficient problem which will appear as a thickness change in the optical system. Although the linear expansion coefficient of quartz (silica) is very small, 5.times.10.sup.-7 /.degree. C., it exhibits a large dn/dT value of +10.times.10.sup.-6 /.degree. C. Because of this, attempts have been made to find a special oxide glass system to make the temperature coefficient of the refractive index, dn/dT, zero, and to account for both the refractive index temperature change and expansion. The development of athermal glass, whose optical path length temperature change, ds/dT, is zero, has been attempted by developing special oxide glasses (F. Reitmayer and H. Schroeder, Appl. Opt., 14 (1975) 716). However, these are based on interference methods utilizing a visible laser light; thus they are athermal glasses in the visible region. So far no report has been published on athermal glasses at wavelengths longer than 1 .mu.m, which would be important for infrared cameras, etc.
Of course, the infrared transparency of metal fluoride glasses has long been recognized, and efforts have been made to develop stable fluoride glasses to exploit this transparency. U.S. Pat. Nos. 4,537,864 and 4,752,593, for example, report stabilized Cd--Li--A1--Pb--F glasses with excellent transparency in the 2-6 micron wavelength range.
Little attention has been given, however, to the thermal properties of halide glasses in the infrared regime. Thus the values of dn/dT and ds/dT of fluoride glasses, have been reported only for ZrF.sub.4 system glasses. These values of dn/dT and ds/dT.sub.abs are, respectively, -11.times.10.sup.-6 /.degree. C. and -2.2.times.10.sup.-6 /.degree. C. (for ZrF.sub.4 --BaF.sub.2 --GdF.sub.3 --AlF.sub.3); and for the AlF.sub.3 system glass example, dn/dT and ds/dT.sub.abs are -6.7.times.10.sup.-6 /.degree. C. and Technology, Vol. 7, No. 8 (1989) p. 1256. These numbers do not suggest that fluoride glasses would be superior infrared athermal glasses, consequently, no report has been published on optical systems and optical parts constructed of athermal glasses operating in the infrared region.
Furthermore, athermal glasses are effective as laser media where the laser medium refractive index temperature change due to heat generation during laser oscillation and the optical path length change present problems for the stability of the laser oscillation mode. In recent years, oxide glasses and fluoride glasses are being developed as media for Er, Nd, etc. lasers which oscillate at wavelengths longer than 1 .mu.m; however, presently, there has been no use of athermal glass as a laser medium at wavelength longer than 1 .mu.m.
As described above, the realization of infrared athermal glasses which will not generate optical path length difference in the infrared region and the realization of optical parts and optical equipment utilizing these glasses are strongly desired.
The objective of the present invention is to provide new glass compositions which do not change optical path length as a function of temperature change in the 1-5.5 .mu.m band. Furthermore, by using these glasses as lens or laser media for incorporation into optical parts, it is an aspect of the present invention to provide infrared optical equipment or optical parts which will not suffer degraded resolution capability due to temperature change in the infrared region and which will not increase in instability during laser performance.